Sky is among the most important subject matters frequently seen in photographic images. In a digital color image, a pixel or region represents sky if it corresponds to a sky region in the original scene. In essence, a pixel or region represents sky if it is an image of the earth's atmosphere. In natural scenes, many objects occlude the view of a portion the sky. These objects can be referred to as sky occluding objects. Examples include banging wires (electrical wires, clothes lines, etc.), airplanes, vapor trails, telephone poles, clouds, etc. Often these objects are made by humans and appear out of context in a natural environment. Many artists use so-called “artistic license” and omit sky occluding objects when painting or drawing natural scenes. However, their methods do not provide a solution for photographers, who would often prefer to capture photographs of scenes as they would appear if the sky occluding objects were absent.
One particularly common class of sky occluding object is hanging wires. A hanging wire is defined as any flexible cord that is fixed at two points and allowed to hang freely in between. Examples include electric wires, telephone lines, chains, ropes, etc.) Hanging wires can be particularly distracting in photographs. Certainly, manual and labor intensive methods can be used to remove hanging wires from photographs. Currently, none of the prior art describes automatic methods for detecting and removing hanging wires from images.
Hanging wires hang in the direction of gravity and therefore provide a clue about the orientation of the image in which they appear. None of the prior art that describes automatically detecting the orientation of an image use this clue. For example, commonly assigned U.S. Pat. No. 5,642,443, entitled, “Whole Order Orientation Method and Apparatus” by Robert M. Goodwin, uses color and (lack of) texture to indicate pixels associated with sky in the image and then surmises the orientation of a whole order of images. It is possible that current methods for detecting the orientation of an image can incorrectly determine the orientation despite the fact that the image may contain a hanging wire that clearly indicates the direction of gravity and the image orientation.